1. Field of the Invention
The present invention relates to an apparatus for and a method of measuring a focus of a television receiver or a monitoring image receiver using a cathode ray tube.
2. Background of the Invention
A television receiver using a cathode ray tube is required to be adjusted to an appropriate condition by measuring the focus of an electron beam so that a satisfactory picture may be displayed on a screen of the cathode ray tube before shipping it after manufacturing process in a factory. In a conventional method of measuring the focus, an operator used to control a level for focus adjustment of the image receiver while checking by his eye a picture displayed on the cathode ray tube of the image receiver. However, such a method of adjustment has an inconvenient drawback in which an accuracy of the focus adjustment depends on a judgement of the operator because he directly views the picture to estimate the focus, so that the accuracy will not tend to be uniform.
In order to overcome this inconvenience, the present applicant previously proposed a method of measuring a focus in which a displayed picture is detected by a sensor so as to enable a correct measurement of a shape of a beam spot (Pat. application No. 4-304032, etc.)
According to this method of measurement, the picture displayed on the screen of the cathode ray tube is made incident onto a CCD line sensor through a rod lens array for measuring the shape of the beam spot from an output of the CCD line sensor. Specifically, for example, as is shown in FIG. 1A, the beam spot emitting a green light is shifted from a position shown by a broken line to a position shown by a solid line for displaying a bright line having a broad width. Here, a position denoted by G shows a position at which a green fluorescent substance is arranged and it is only this portion that actually emits light. Accordingly, the fluorescent substance emits light in a manner that each position of the green fluorescent substance emits light with a brightness corresponding to the beam spot that passed therethrough, as is shown in FIG. 1B.
If the emitted light of the green fluorescent substance is detected by the line sensor, there is provided an output which is almost proportional to the light emission of the fluorescent substance, as shown in FIG. 1C. Since the output of the line sensor includes a noise component, the output from the line sensor is caused to pass through an electric low pass filter (e.g. a filter formed of a capacitor and a resistor) for obtaining a detected output without the noise component, as shown in FIG. 1D. The bright line width is measured from this detected output and then a processing for measuring a diameter of the beam spot is performed based on that result. By measuring according to the process as shown in FIGS. 1A to 1D, a uniform measurement can be made without any influence from a change of the light emission due to a positional relationship between the fluorescent substance and the beam spot.
However, a ripple is introduced in the output of the line sensor that passed through the low pass filter. Thus, a ripple component L is generated in the output of the low pass filter, as is shown in FIG. 1D. If this ripple component is large, the ripple component itself can be decided to be the detected output of the beam, thereby disturbing the measurement of the bright line width.
In order to eliminate this ripple component, it may be enough to set a passing frequency fc of the low pass filter to an appropriate value. However, since the bright line width of the cathode ray tube is different due to difference of a size of the screen or a mode of scanning of the image receiver to be measured, it is necessary to change the passing frequency fc of the low pass filter whenever the bright line widths of the cathode ray tube differ, which in turn brings an inconvenience in which a configuration of the detector circuit becomes extremely complicated.